Man, Arcteryx makes nice stuff. I've never been disappointed in anything with their name on it and the SL is no exception. Its much lighter than the Atom LT and offers a lot less warmth and a lot less bulk. Its basically the same perfect jacket but for days that aren't so cold. I found that a nice base layer under the SL works beautifully even on cold days, too, though. In fact, layering is really the way to go and a super light, comfy and with a little synthetic warmth makes the Atom SL perfect.
This review was written in the old system and had content requirements that are different than reviews written today.